President Barack Obama warned Wednesday that the United States was ready to respond to China's "aggression" toward its neighbors at sea but said Washington should lead by example by ratifying a key treaty.

In a wide-ranging speech on foreign policy to US military cadets at West Point, Obama said that the United States should shun isolationism and that its military must be prepared for crises.

"Regional aggression that goes unchecked -- whether it's southern Ukraine, or the South China Sea, or anywhere else in the world -- will ultimately impact our allies, and could draw in our military," Obama said.

But Obama emphasized caution on any decision to use force and said: "American influence is always stronger when we lead by example."

"We can't try to resolve the problems in the South China Sea when we have refused to make sure that the Law of the Sea Convention is ratified by the United States -- despite the fact that our top military leaders say that the treaty advances our national security," Obama said, not naming China directly as he diverted from his prepared text.

"That's not leadership; that's retreat. That's not strength; that's weakness," Obama said.

Senators of the rival Republican Party have refused to ratify the treaty, saying that the UN convention would override US sovereignty.

Tensions have been rising for months between China and its neighbors at sea, with Vietnam on Tuesday accusing Beijing of ramming and sinking one of its fishing boats in the South China Sea.

Japan and the Philippines also have tense disputes at sea with China. Japanese commentators have voiced concern that the US failure to prevent Russia from annexing Ukraine's Crimea peninsula in March sent the wrong signal to China.

In another reference to policy toward Asia, Obama again cited the democratic reforms in Myanmar as a success story.

The administration upon entering office in 2009 opened a dialogue with the then military-ruled nation earlier known as Burma, whose relations have improved with the United States have improved dramatically.

"Progress there could be reversed. But if Burma succeeds, we will have gained a new partner without having fired a shot," Obama said.

Myanmar has freed political prisoners, eased censorship and welcomed foreign investors, but human rights groups have voiced alarm over violence against the mostly Muslim Rohingya minority.